Research in a wide variety of areas of pure and applied science is conducted by a group of students, engineers, senior visitors, and faculty members. Campus facilities include a number of ultra-high vacuum (UHV) ion implantation devices, materials-characterization apparatus, an EN-tandem accelerator, and two IBM RS-6000 work stations. There are off-campus collaborations at California State University at Fullerton, JPL, Schlumberger-Doll Research, Uppsala University, and Universite' Paris (Sud).
In general, the group is engaged in applying the techniques of theoretical and experimental physics to problems in materials science, surface physics, and planetary science.
There has been a long-term interest in understanding the damage processes caused by MeV ions in solids. This is motivated both by the desire to understand defect formation at a fundamental level and also by the utility of using such effects in the modification of the near-surface regions of electronic materials. An example of the former is our studies of mechanisms of damage-track formation in III-V semiconductors and yttrium ion garnet; in the case of the latter we exploit our ability to produce damage structures in a controlled way to study (with N.-C. Yeh's group) the properties of high-Tc superconductors.
Sputtering and Secondary-Ion Emission
To improve the precision of secondary-ion mass spectrometric analyses, a basic program has been developed to study the sputtering of materials by low energy ions. This research involves both experimental studies and molecular dynamics computations that connect the observations with the atomic interactions involved. Recent work has explored the basic mechanisms by observing isotopic fractionation produced in the eroded materials. We have also demonstrated clearly for the first time the depth of origin of the ejected atoms by using liquid-metal alloy targets in which Gibbsean segregation produces and maintains a monolayer of atoms at the surface that is very different in composition from the layers below it.
In addition to the obvious practical applications of these processes in materials analysis and modification, there is a well-established program in how such ion-surface processes occur in the interstellar medium as well as in planetary environments, where they redistribute material on the surfaces of natural satellites and modify their tenuous atmospheric coronae.
In addition to single-ion impacts, considerable effort has gone into understanding the effects produced by large molecular-ion or cluster-ion bombardment. We have pioneered the use of molecular-dynamics simulations to understand better these collisions and have established strong collaborations with experimental groups in Sweden and France to verify the predictions of the calculations and utilize such processes in technological applications (e.g., SIMS, thin-film deposition, micromachining).
In collaboration with Schlumberger's research laboratory (Ridgefield, CT) we are growing and studying novel light-emitting materials. One example is the rare-earth oxyorthosilicates which are being used in fields as diverse as cathode-ray tubes, positron-emission topography, oil/gas-well logging, and planetary missions. Using unique facilities at Schlumberger and Caltech as well as the synchrotron-light source at Brookhaven National Laboratory we are studying the basic light-emission mechanisms in these materials as well as their applications.
We are involved in collaborations with JPL and Goddard Space Flight Center to exploit the unusual properties of scintillators grown from these materials in proposed missions for Venus, Mars, and cometary landers.
Experimental Investigations of the critical Vortex Dynamics in Extreme Type-II Superconductors with controlled Static Disorder, N.-C. Yeh, W. Jiang, D. S. Reed, U. Kriplani, M. Konczykowski, T. A. Tombrello, F. Holtzberg, and C. C. Tsuei, Ferroelectrics 177 (1996) 142-159.
Atomic Diffusion in the Supercooled Liquid and Glassy State of the Alloy Zr_41.2Ti_13.8Cu12.5Ni_10Be_22.5 U. Geyer, S. Schneider, W. L. Johnson, Y. Qiu, T. A. Tombrello, and M.-P. Macht, Phys. Rev. Lett. 75 (1995) 2364
Molecular Dynamics Simulations of Inner Shell Electronic Energy Losses in Cluster-Surface Collisions, S. J. Timoner, M. H. Shapiro, and T. A. Tombrello, Nucl. Instr. & Meth. B114 (1996) 20-27
Vortex Pinning by Cylindrical Defects in Type-II Superconductors - Numerical Solutions to the Ginzburg-Landau Equations, S. M. Maurer, N.-C. Yeh, and T. A. Tombrello, Phys. Rev. B54 (1996) 15372-15379
Ion Beam Mixing Induced by Atomic and Cluster Bombardment in the Electronic Stopping Power Regime, M. Beranger, P. Theuenard, R. Brenier, B. Canut, S. M. M. Ramos, A. Brunelle, S. Della Negra, Y. Le Beyec, E. Balanzat, and T. Tombrello, Phys. Rev. B53 (1996) 14773-14781
The Role of Cerium Sites in the Scintillation Mechanism of LSO,
J. D. Naud, T. A. Tombrello, C. L. Melcher, and J. S. Schweitzer,
IEEE Trans. on Nucl. Sci. 43(3) (1996) 1324